Fazli, the Iranian interior minister, also issued a warning on 7 September, in which he said "Daesh"—another term referring to Isis—"is poised to attack Iran imminently."

On 13 May, less than 24 hours after Isis spokesperson, Abu Mohammad al Adnani was heard saying that they want to, "transform Iran into pools of blood," a suicide bomb truck claimed the lives of five Iranian engineers.

While Al-Qaeda was known to have complex relations with Iran, Isis too has identified Iran as its "bitterest enemy."

Isis, which reportedly formed out of an Al-Qaeda branch in Iraq, "held back its soldiers and repressed its rage over the years to preserve the unity," of Al-Qaeda.

Iranian Minister of Intelligence Mahmoud Alavi said in October: "As a result of tens of intelligence operations, more than 130 individuals believed to be key members of Takfiri [infidel] groups were arrested in past few months."

'Takfiri' is a term referring to Isis and other Sunni extremist groups.

While Adnani has never outrightly threatened Iran, and attacks like the 13 May explosion are blamed on Isis and other Sunni extremist groups, the American Intelligence community remains sceptical.

"While no one is ruling out the possibility of an ISIL [Isis] presence in Iran... at this time we are not able to validate reports of any activity there," said a US intelligence official to the Daily Beast magazine.